1. Field of the Invention
The present invention relates to an insulation film which is good in terms of the heat resistance, a powder for a magnetic core, powder which is covered with the insulation film, a powder magnetic core which is composed of the magnetic powder, and processes for producing them.
2. Description of the Related Art
Around us, there are many articles, such as transformers, motors, generators, speakers, induction heaters and a variety of actuators, which utilize electromagnetism. Many of these articles utilize alternating magnetic fields. In order to efficiently produce alternating magnetic fields which are great locally, magnetic cores (or soft magnets) are usually disposed in the alternating magnetic fields.
From the nature of the characteristics, those magnetic cores are first required to produce a large magnetic flux in alternating magnetic fields. Next, when they are used in alternating magnetic fields, they are required to exhibit less iron loss which is generated in accordance with the frequencies of the alternating magnetic fields. As the iron loss, there are eddy current loss, hysteresis loss and residual loss. Among them, however, the eddy current loss and the hysteresis loss matter mostly. Moreover, in order that magnetic cores follow magnetic fields to produce high magnetic flux density, it is important as well that their coercive forces are small. Note that it is possible to improve (initial) magnetic permeability and reduce hysteresis loss at the same time by reducing the coercive forces.
However, it is difficult to simultaneously satisfy those requirements. Not to speak of simple iron cores, but conventional cores in which thin silicon steel plates are laminated have not produced sufficient performance. Accordingly, it has been a trend recently to solve the problems by using powder magnetic cores which are formed by pressurizing magnetic powders (or magnetic core powders) covered with insulation films. Specifically, the respective particles of magnetic powders are covered with insulation films to enlarge the specific resistance, and thereby the iron loss of powder magnetic cores is reduced. At the same time, such powders are formed with high pressures to produce powder magnetic cores with a high density, and thereby it is intended to enlarge the magnetic flux density. Such a powder magnetic core is disclosed in PCT International Laid-Open Publication No. 2000-504,785, for example. According to the publication, a pure iron powder being a magnetic powder is contacted with a phosphoric acid solution to generate an insulation film being composed of a phosphate (or iron phosphate) film on a surface of the pure iron powder. The resulting powder is formed by pressurizing to make a powder magnetic core.
However, it is not yet possible to say that powder magnetic cores so far have had sufficient performance. The reasons are as follows. Above all, since magnetic powders are formed at low pressures, which are determined while taking the longevity, and the like, of molds into consideration, the resulting conventional powder magnetic cores have a low density so that they cannot produce a sufficiently high magnetic flux density.
Yet, the applicants (or assignees) of the present invention have been already solved in this regard. That is, they have already developed technologies which make it possible to form magnetic powders with super high pressures, and have succeeded in producing powder magnetic cores, which are highly densified approximately to the true density, from magnetic powders which are covered with insulation films. Moreover, they have already filed a plurality of patent applications for the technologies.
Another reason why the performance of conventional powder magnetic cores is insufficient is that the iron loss cannot be reduced sufficiently by simply disposing insulation films on a surface of magnetic powders. Specifically, among the iron loss, in particular, the eddy current loss has been reduced so far by enlarging the specific resistance mostly. Accordingly, it has not been intended so much to reduce the hysteresis loss itself. Of course, the hysteresis loss does not matter in powder magnetic cores which are used in a frequency range (or a super high frequency range) where the hysteresis loss is negligible compared with the eddy current loss. However, many articles are often used in a frequency range of some hundreds Hz or less, for example. In such a frequency range, it is not possible to ignore even the hysteresis loss in powder magnetic cores.
As described above, in order to reduce the hysteresis loss in powder magnetic cores, it is effective to reduce the coercive force of powder magnetic cores. The coercive force is influenced by strain which resides in the particles of magnetic powders. The greater the strain is, the greater the coercive force is. In view of producing powder magnetic cores, it is inevitable that residual strain arises more or less in the particles of magnetic powders after they are formed by pressurizing. Therefore, in order to reduce the hysteresis loss, it is necessary to remove the residual strain which arises once in the particles of magnetic powders. In order to remove the residual strain, it is effective to subject powder magnetic cores to a heat treatment such as annealing for removing residual stress.
The heat treatment depends on the types of magnetic powders. However, in the case of ordinary magnetic powders in which Fe is a major component, it is desirable to heat them at 450° C. or more, further to about 500° C., to fully remove the strain residing in them.
However, when powder magnetic cores are heated to such high temperatures, the resinous films, which have been used conventionally as insulation films for magnetic powders, are decomposed to disappear. Even the above-described phosphate film (or chemical film) crystallizes to cause sintering and agglomerating. Thus, it has become apparent that the crystallized phosphate films concentrate at the spaces (or triple point) formed among the particles of magnetic powder, and magnetic powders react with insulation films to destroy insulation films. When such is the case, the specific resistance is reduced sharply to enlarge the eddy current loss, and to adversely result in increasing the iron loss. Thus, it is meaningless to carry out the heat treatment. Here, it is possible to think of using oxide-based, such as SiO2, Al2O3, ZrO2 and TiO2-based, insulation films whose relatively heat resistance is high. However, it is difficult technically to coat a thin oxide film whose thickness is some dozens of nanometers on magnetic powders without pinholes. Moreover, since the costs are high remarkably, such a method is not effective industrially. On the other hand, when the oxide-based films are thickened to 100 nm or more, such a method is not preferable after all because the resulting powder magnetic cores exhibit a lowered magnetic flux density.
Hence, in Japanese Unexamined Patent Publication (KOKAI) No. 6-132,109 and Japanese Patent Publication No. 2,710,152, for example, there are disclosures about glassy insulation layers in which chromium (Cr) and magnesium (Mg) are requisite constituent elements to enhance the heat resistance.
However, as set forth in Japanese Patent Publication No. 2,710,152, it is not preferable to use Cr in view of environment. Moreover, according to the survey and study conducted by the present inventors, it seemed that the glassy insulation layers in which Mg is a requisite constituent element were surely improved in terms of the heat resistance compared with conventional ones. However, the heat resistance was not yet sufficient at all.